1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device and, more particularly, to an organic EL display device in which organic layers have improved adhesion to each other.
2. Discussion of the Related Art
Generally, an organic EL device includes an anode, a cathode, and an organic layer. The organic layer may be comprised of various layers, including a hole injection layer, a hole transport layer, an emission layer, an electron transport layer, and an electron injection layer. The organic EL device may be classified as a polymer type and a small-molecule type according to materials used in the organic layer. For the small-molecule organic EL device, each layer may be formed by a vacuum deposition method, and for the polymer organic EL device, the layers may be made by a spin coating process.
Single color organic EL devices using polymer may be simply fabricated by the spin coating process. Although the polymer organic EL device has a lower driving voltage than the small-molecule device, it is less efficient, and it has a shorter lifetime. Further, patterning red, green and blue polymers for a full-color display device by inkjet technology or a laser induced thermal imaging (LITI) method may degrade emission characteristics such as efficiency and lifespan.
When the patterning by the LITI method, single polymer materials may not transfer. Korean Patent Application No. 1998-51844 and U.S. Pat. Nos. 5,998,085, 6,214,520 and 6,114,088 disclose a method of forming the pattern of the polymer organic EL device by the LITI method.
The LITI method may utilize a light source, a transfer film and a substrate. A light absorption layer of the transfer film absorbs emitted light from the light source and converts it into thermal energy. Material forming a transfer layer of the transfer film is then transferred onto the substrate by the thermal energy to form a desired image (U.S. Pat. Nos. 5,220,348, 5,256,506, 5,278,023 and 5,308,737).
As disclosed in U.S. Pat. No. 5,998,085, the LITI method may be used to fabricate a color filter of a liquid crystal display device, and it may also be used to form a pattern of emission material. This method may transfer the material without transferring heat, unlike the LITI method using sublimation of the material.
U.S. Pat. No. 5,937,272 discloses a method of forming high definition patterned organic layers in a full-color organic EL device, wherein a donor support coated with a coating material is heated to transfer the organic EL material onto a designated recessed surface of a substrate in a designated sub-pixel. Heat or light may vaporize the emission material, which transfers it onto the pixel.
Similarly, in U.S. Pat. No. 5,688,551, a transfer from a donor sheet to a receiver sheet may be used to form a sub-pixel in a pixel region. An organic EL material with sublimation properties may be transferred from the donor sheet to the receiver sheet at a low temperature (less than 400° C.) to form the sub-pixel.
FIG. 1 is a cross-sectional view showing a typical structure of a conventional full-color organic EL device.
Referring to FIG. 1, a first electrode 110 is patterned and formed on an insulating substrate 100. The first electrode 110 may be transparent for a bottom emission structure, and it may be a reflective for a top emission structure.
A pixel defining layer (PDL) 120 may be formed of an insulating material on the first electrode 110 and the substrate 100 to define a pixel region and to insulate between emission layers.
An organic layer 300 having an organic emission layer 150 may be formed in the pixel region defined by the PDL 120. The organic layer 300 may further include a hole injection layer 130, a hole transport layer 140, a hole blocking layer 160, an electron transport layer 170 and an electron injection layer 180, as well as the above organic emission layer. Both polymer and small-molecule materials may be used as the organic emission layer 150.
A second electrode 190 may be formed on the organic layer 300. The second electrode 190 may be reflective when the first electrode 110 is transparent, and it may be transparent when the first electrode 110 is reflective. Encapsulating the organic EL device completes its fabrication.
However, when an organic layer having a conventional organic emission layer is formed by the LITI method, adhesion between a transfer layer material to be transferred from a donor film and an organic material formed on the substrate may not be sufficient. If the materials do not adhere appropriately, particles may be generated by breakage and exfoliation of the organic layer, and the organic layer may crack.